1. Field of the Invention
The invention relates to a new method of starting up loop reactor systems in which at least one exothermic reaction is to be carried out.
2. Discussion of the Background
Loop reactor systems are frequently used for carrying out exothermic reactions since the reaction can be controlled in a simple manner via the amount of product which is circulated.
A typical reaction that can be carried out in a loop reactor is the acid catalyzed cleavage of cumene hydroperoxide (CHP) into phenol and acetone. Phenol is a valuable basic organic chemical. It is used, for example, for the production of bisphenol A, .epsilon.-caprolactam, adipic acid, alkylphenols, chlorophenols, antioxidants, plasticizers and phenolic resins. The annual worldwide capacity for the production of phenol is about 6 million metric tons (Weissermehl, Arpe, Industrial Organic Chemistry, Third Edition, 1997, VCH Weinheim).
The most important method of preparing phenol is the oxidation of cumene wherein cumene hydroperoxide is first formed and then cleaved in the presence of an acid catalyst to form phenol and acetone. This cleavage is preferably carried out in a loop reactor system, with the loop reactors being run at an optimum operating temperature of 50 to 90.degree. C.
EP 0 589 588 discloses a process for preparing phenol, acetone and a methylstyrene that uses a loop reactor system having three loop reactors for the cleavage of CHP. The operating temperatures of the loop reactors reported for this process are 50 to 62.degree. C. for the first loop reactor, 62 to 57.degree. C. for the second loop reactor and 57 to 50.degree. C. for the third loop reactor.
The procedure that is typically used for starting up such loop reactor systems is, e.g., in the cleavage of CHP, to circulate a reaction mixture that predominantly includes the products and the catalyst through the loop reactor system and then heat the system to the reaction temperature. This is usually done by switching over one of the heat exchangers in the loop reactor system from a coolant to a heat transfer medium such as steam while interrupting the coolant flow through any other heat exchangers present in the system. The steam, which transfers its heat to the circulating reaction mixture in the heat exchanger, allows the reaction mixture to be heated to the start up temperature. This is supposed to ensure that the exothermic reaction does not continue in the downstream systems without sufficient cooling.
The above method of starting up loop reactor systems in which an exothermic reaction is to proceed is relatively complicated since at least one heat exchanger must be switched over from coolant to steam, and the coolant must first be removed from the heat exchanger before the steam can be passed into the heat exchanger. If there are any leaks in the shut-off valves for the coolant in the heat exchangers, which have only been turned off and from which the coolant has not been removed, the heating-up process may take a relatively long time. For this reason, comprehensive additional technical facilities become necessary.
In the above-noted method for heating up the reaction mixture in a loop reactor system, it is also possible for the products present in the reaction mixture, e.g. phenol and acetone, to react with the catalyst present in the reaction mixture to form undesired by-products during the long heating-up time, which undesirably results in the loss of product.
In addition, when steam is supplied to the heat exchanger, the temperature at the heat exchanger can rise above the temperature that normally prevails during the reaction so that when sulfuric acid is used as catalyst, e.g. in the cleavage of CHP, the heat exchanger is exposed to an increased corrosive stress owing to the high temperature.